An important aspect of most methods for compressing sequences of digital video images is motion analysis. In typical motion analysis, a target image in a sequence of images is divided into a plurality of blocks of pixels. A target region comprising at least one pixel block is selected from the target image. The image preceding the target image is analyzed in order to locate a region of the preceding image which closely matches the target region of the target image. Block matching techniques based on mean-square-error or mean-absolute-error are typically used to locate the region of the preceding image which closely matches the target region of the target image. If no motion has occurred, the preceding and target regions will have the same coordinates within the image frame. If motion has occurred, the preceding region will be offset or translated relative to the target region by an amount which is represented by a displacement vector. This process is typically repeated for each target region in the target image, thereby yielding a displacement vector for each target region. These displacement vectors are thereafter applied to a previous reconstructed image to form a predicted image. An error image is formed from the difference (residual errors) between the target image and the predicted image. The displacement vectors and the error image are then encoded for subsequent use by a decoder in decoding the compressed digital video signal. A known motion compensation system is described in U.S. Pat. No. 5,134,478 to Golin, entitled "Method And Apparatus For Compressing And Decompressing A Digital Video Signal Using Predicted And Error Images", the contents of which is hereby incorporated herein by reference.
In image compression, image quality can be improved by encoding images unequally. More particularly, the residual errors that result from the application of motion compensation can be encoded unequally from frame to frame in order to improve image quality. Such an encoding technique is employed in MPEG encoding, which encodes residual errors in every third image more accurately than in intermediate images. This can be done by varying the quantization. An alternative approach, set forth by the present application, applies heuristics to the residual errors in order to determine which ones may be ignored. A heuristic is a rule which produces a visually acceptable image when measured using subjective criteria (such as how the image looks to a viewer), as opposed to an objective measurement such as mean-square-error.
It is an object of the present invention to improve image quality by using heuristics to unequally encode residual errors, such as those that result from motion compensation.
Further objects and advantages of the invention will become apparent from the description of the invention which follows.